1. Field of the Invention
The invention relates to a quasi-optical gyrotron in which two coils in a Helmholtz arrangement generate a static magnetic field which is axially symmetrical with respect to an electron beam axis, electrons passing along the electron beam axis parallel to the magnetic field are forced into gyration and excite an alternating electromagnetic field in a quasi-optical resonator which comprises two mirrors arranged opposite to one another on a resonator axis, the resonator axis being aligned perpendicularly to the electron beam axis between the two coils.
2. Description of Background
A quasi-optical gyrotron of the type initially mentioned is known, for example, from Patent CH 664045 or from the article "Das Gyrotron, Schlusselkomponente fur Hochleistungs-Mikrowellensender" (The gyrotron, key component for high-power microwave transmitters), H. G. Mathews, Minh Quang Tran, Brown Boveri Review 6-1987, pages 303-307.
While the microwave power of such gyrotrons is still limited at present to a few 100 kW at operating frequencies of more than about 100 GHz, it should be possible to generate continuous-wave powers of 1 MW and more, having regard to applications in plasma heating for fusion purposes.
The unwanted heating-up of the resonator walls represents one problem in achieving such high-power gyrotrons. This is because, due to the finite electric conductivity of the walls, these walls are heated up by the RF field in the resonator. This limits the achievable microwave power due to the maximum heat loss which can be dissipated.
Increasing the conductivity of the walls lowers the heat loss and correspondingly improves the power capability of the gyrotron. In view of the fact that, on the one hand, normally-conductive metals have hitherto been used exclusively for the resonator and that, on the other hand, high temperature superconductors have recently become available, it appears to be obvious to replace the normal conductors by superconductors in the resonator. This has also been proposed already in the literature (see, for example, "Possibilities for microwave/far infrared cavities and waveguides using high temperature superconductors", D. R. Cohn, L. Bromberg, W. Halverson, B. Lax and P. Woscov, Twelfth International Conference on Infrared and Millimeter Waves, Dec. 14-18, 1987, Lake Buena Vista, Fla., Conference Digest, IEEE Catalog No. 87CH2490-1, page 51-52).
In this connection, the problem is that in a quasi-optical gyrotron the superconductors must operate both in the presence of RF fields (&gt;100 GHz) and in the presence of strong magnetic fields (&gt;5 T). Under these conditions, however, all known superconductors have worse electrical characteristics than copper and thus do not offer any advantages.